1. Field of the Invention
The present invention relates generally to a mobile communication system supporting multiple users by use of multiple transmit/receive antennas, and in particular, to an eigen-based scheduling method and apparatus which are robust against spatial correlation.
2. Description of the Related Art
Conventional TDM (Time Division Multiplexing) or CDM (Code Division Multiplexing) mobile communication systems are not suitable to support future-generation high-speed multimedia transmission because they were developed to service low-speed data, which mainly consist of voice data. To realize high-speed transmission, there is a need to develop techniques to improve the efficiency of use of the limited frequency resources. A main technique proposed as a solution is MIMO (Multiple Input and Multiple Output) using multiple transmit/receive antennas.
A multi-transmit/receive antenna system applies an appropriate space-time signal processing scheme to a transmitter/receiver using a plurality of transmit/receive antennas. The resulting increase in efficiency of frequency use allows a high data rate with a limited bandwidth. It is known that a maximum available data rate conforms with the capacity of a radio channel that delivers a signal and the theoretical capacity of the radio channel for which a sufficient number of paths exists is approximately proportional to the number of the transmit/receive antennas.
A single-antenna TDM mobile communication system uses channels only in the time domain because one frequency channel is divided into a plurality of time slots. On the other hand, a multi-transmit/receive antenna system forms a plurality of sub-channels in the spatial domain and transmits different data simultaneously on the sub-channels, irrespective of whether a transmitter needs channel information. The sub-channels are equivalent to radio paths from a plurality of transmit antennas to a receiver. With this characteristic, the multi-transmit/receive antenna system can achieve a higher data rate than the single-antenna system.
Past studies on multi-transmit/receive antenna systems have focused mainly on development of techniques for increasing data transmission efficiency or reducing transmission errors in a single-user system with a single transmitter and a single receiver.
Since multiple user terminals are mutually independent in a geographical location and propagation environment, radio channels between a base station (BS) and the user terminals are also independent of each other. Therefore, there may exist users in a channel environment suitable for data transmission in a particular time (i.e. time slots) as well as users in a channel environment not suitable for data transmission due to fading, for example. Considering the channel independency, the BS assigns transmit antennas to optimum users in each time slot, to thereby maximize system capacity. This operation is called scheduling. Scheduling is a very complicated task involving diverse factors such as the varied channel environments of the user terminals, the amount of data to be transmitted, and priority levels.
FIG. 1 illustrates an assignment of space and time by MIMO scheduling in a multi-user environment. In the example of FIG. 1, four antennas are assigned among different users in each time slot.
The main point of MIMO scheduling is how to assign transmit antennas on a BS side and how to detect signals from the BS on the user terminals side.
FIG. 2 illustrates the configuration of a MIMO mobile communication system that schedules in a typical manner. M users are selected among a total of K users (M is the number of transmit antennas).
Referring to FIG. 2, a BS is servicing, i.e. providing data, to K users. The BS transmits data to M user terminals 40 to 44 through M (M<K) antennas 22 in one time slot. That is, a scheduler 20 selects user data for the M user terminals 40 to 44 under optimum channel environments from K user buffers 10 to 14 and transmits the user data to the selected user terminals through corresponding antennas.
Traditionally in the scheduling, an optimum user is selected for each transmit antenna based on the SNRs (Signal to Noise Ratios) of respective transmit antennas fed back as channel information from all users receiving data service. When the BS provides a data service to K users each using N receive antennas through M transmit antennas, feedback information from a kth user to the BS is described by the following Equation 1:
                                                                                          γ                  k                                =                                                      [                                                                                                                        γ                                                          1                              ,                              k                                                                                                                                                            γ                                                          2                              ,                              k                                                                                                                                …                                                                                                      γ                                                          M                              ,                              k                                                                                                                                            ]                                    T                                            ,                                                                          k                =                1                            ,              2              ,              …              ⁢                                                          ,              K                                                                                          =                                  1                                                            [                                                                        H                          k                          H                                                ⁢                                                  H                          k                                                                    ]                                        min                                          -                      1                                                                                  ,                                                                          m                =                1                            ,              2              ,              …              ⁢                                                          ,              M                                                          (        1        )            where γij denotes the SNR of a channel from an ith transmit antenna to a jth user and superscript H denotes a Hermitian matrix. The scheduler 20 selects users having the best SNRs for the respective transmit antennas in accordance with the following Equation 2:dm=arg maxk(γm,k)  (2)where dm is the index of a user selected for an mth antenna. The scheduler 20 then selects data Sd1, Sd2, . . . , sdM for the selected users and transmits them to the M transmit antennas through a multiplexer (not shown).
The kth user estimates a channel matrix Hk representing channel characteristics from the transmit antennas to the receive antennas and detects the data Sdk transmitted by the transmitter by a detection algorithm, such as ZF (Zero-Forcing). A ZF receiver independently detects data simply through the inversion of the channel matrix, thereby reducing receiver complexity.
Under the assumption that a data stream transmitted through the M transmit antennas is an M×1 signal vector and a matrix representing the characteristics of a radio channel 30 that delivers the transmit signal vector is H, a receiver with N receive antennas receives an N×1 signal vector r that can be expressed in accordance with the following Equation 3:r=Hs+w   (3)where w denotes Gaussian noise which is an (N×1) vector since it is induced to each receive antenna and H is an N×M matrix because signals transmitted from the M transmit antennas arrive at the N receive antennas via different paths.
However, when the channel spatial correlation is high, for example, when the transmit antennas are widely spaced, the rank of the channel matrix is reduced. As a result, there is no inverse matrix, or if there is an inverse matrix, each element in the inverse matrix has a very great value. The matrix rank refers to the number of independent column pairs.
Let rk denote a signal vector received at the kth user from an mth transmit antenna. The kth user then detects data in accordance with the following Equation 4:
                                                                                          s                  ^                                                  k                  ,                  m                                            =                                                                                                                  E                        s                                            M                                                        ⁢                                      s                                          k                      ,                      m                                                                      +                                                      [                                                                  H                        k                                                  -                          1                                                                    ⁢                                              n                        k                                                              ]                                    m                                                                                                        =                                                [                                                            H                      k                                              -                        1                                                              ⁢                                          r                      k                                                        ]                                m                                                                        (        4        )            where Sk,m is the transmitted data, ŝk,m is the detected data, and nk is a noise vector introduced to the kth user. Therefore, the variance of [Hk−1nk]m is N0[HkHHk]mm−1. As noted, when the values of elements in the inverse of the channel matrix increase with spatial correlation, the effects of noise also become serious.
FIG. 3 is a graph illustrating ergodic capacity versus spatial correlation in a simulated conventional scheduling method. The simulation was performed under the conditions of four transmit antennas, four receive antennas, 50 users, and an SNR of 20 dB. As illustrated, in the conventional scheduling, as spatial correlation increases, the power of noise in a detected signal also increases. Thus, system performance rapidly deteriorates.